Studies are planned to examine the development of anti-Trypanosoma cruzi antibody and lymphocyte mediated cytotoxic responses in experimental Chagas' disease in resistant (C57BL/6) and susceptible (C3H) mice. Both antibody and cellular mediated cytotoxic responses will be assayed using a 3H-thymidine terminal labeling procedure for post-assay detection of viable parasites. Attempts will be made to correlate the role of humoral and cellular immunosuppression to development of resistance to T. cruzi. Of particular interest will be examination of a host-specific immunosuppressive substance found in the serum of infected mice and a parasite eleborated substance which decomplements normal serum. The molecular size and class of each substance will be elucidated and the mode of action and the role of the suppressor substance will be examined.